Economic irrigation of growing agricultural crops is of increasing importance. In many drier regions, water availability is being continually reduced. As a result, the water that is available must be ever more carefully used by farmers in order to maintain crop yield. Even in areas where water for irrigation is in plentiful supply, irrigation needs to be carefully planned to minimize costs and to avoid overwatering. Excessive watering can erode soil and leach valuable plant nutrients from the soil.
For many years, irrigation of agricultural crops has been performed in a relatively unsophisticated manner. For example, some crops are periodically watered at fixed intervals for an established time. Other crops are watered so as to maintain a particular soil moisture level. These procedures do not prevent crop damage from over or underwatering, excessive water use, nor minimize expenses.
In an effort to provide guidelines for more efficient and effective irrigation of crops, various groups of soil, crop and field conditions, such as soil moisture content, crop and air temperature, relative humidity, sunlight intensity and water flow have been monitored for various growing crops. Researchers have attempted to correlate the results of these measurements with crop growth and yield. It has been a goal of these studies to devise one or more indices of crop water stress that indicate whether or not a crop needs irrigation. If such a measurement were available, a crop could be irrigated only when the measured stress indicated that adverse consequences may result should the crop not be watered.
Crop water stress can be measured in various ways. One technique is the measurement of crop canopy (leaf) temperature in relationship to the air temperature. When a leaf is freely transpiring, the cooling properties of the evaporating water generally keep the leaf temperature below that of the air. When the plant water intake becomes deficient, for example, when soil moisture content falls, the temperature of the leaf increases because transpiration, convection and thermal radiation dissipate less of the heat load than when plant water intake needs are met. In that case, the leaf temperature will approach, and sometimes exceed, air temperature. However, the difference between the crop canopy temperature and air temperature alone does not universally disclose water stress, since leaf temperature varies with the intensity of the sun at the leaf, the relative humidity of the air surrounding the crop and the crop itself.
Another factor that influences crop water stress is the so called vapor pressure deficit. The vapor pressure deficit is the difference between the vapor pressure of water in the air and the maximum vapor pressure of water that could be supported in air at that temperature. Vapor pressure deficit is closely related to relative humidity. Agricultural researchers found that, for well watered crops in arid climates, the difference between crop canopy and air temperatures is a predictable function of vapor pressure deficit. Further research showed this relationship applied in both dry and humid environments. (Other indices of water stress, such as the difference between the crop canopy and air temperatures alone, while useful in many circumstances, are not reliable indicators of crop water stress in humid environments.) While the expected relationship between temperature differential and vapor pressure deficit for a well watered crop is generally observed for most crops, the precise mathematical relationship between the variables differs from crop to crop, e.g., wheat, corn, potatoes and soybeans. The precise relationship for a particular well watered crop can be determined experimentally. Once that relationship is known, field measurements of crop canopy-air temperature differential and vapor pressure deficit can be made and compared to the experimentally determined relationship. If, for a given vapor pressure deficit, the temperature differential exceeds that for a well watered crop, the crop can be said to be water stressed. The degree of water stress can be determined by the amount the temperature difference exceeds that expected for a well watered crop at the measured vapor pressure deficit. When water stress exceeds an established threshold, irrigation (or some other remedy if the cause of water stress should happen not to be the result of a moisture shortage) is called for to avoid loss of crop yield.
The advances in understanding and quantifying crop water stress have enabled researchers to detect its onset and to predict its effects. Generally, these advances in research have been achieved by academic institutions and government agencies using complex and expensive instrumentation and analysis techniques. Their research has now matured so that its results could be routinely applied by farmers if they could be equipped with practical and economical instruments for determining the water stress of their growing crops. With such an instrument, a farmer can allocate water and energy resources amongst his crops so as to conserve water and minimize expenses while avoiding unnecessary losses in crop yields.